To simplify the stretching work of a belt by fixedly arranging an air compressor on a rack, movably arranging a motor base against the rack, and installing between the motor base and the rack, an adjusting bolt by which the motor base is fixed by being adjusted its movement.

The mounting plate 34 of an air compressor 1 is tightened on the top plate 30a of a rack 30 through a bracket 56 by a bolt 40. The supporting base 42 of a motor 28 is tightened on the mounting plate 41 by a bolt 43, and a vertical wall 30b is sandwiched between the mounting plate 34 and a slide plate 44. The ends of bolts 48 which are passed through bolt holes 49 opened to the slide plate 44, oblong holes 50 opened to the vertical wall 30b and bolt holes 51 opened to the mounting plate 41, are screwed in nuts 52. Two adjusting bolts 45 passed through bolt holes 49 opened to the top plate 30a are passed through bolt holes 55 opened to a flange 44a. Hereby, when a belt 31 is loosened, the adjusting bolts 45 are unscrewed after the bolts 48 are loosened and the nuts 47 are unscrewed, and a motor 28 is lowered while being kept horizontally.